Instead of element isolation technologies based on the local oxidization of silicon (LOCOS) method, a trench filled type element isolation technology has been developed which provides an increase in the capacity of the elements. According to such technology, after an isolation trench is formed on a semiconductor substrate by etching, an insulating film is deposited on a main surface of the semiconductor substrate by a CVD method. By filling the isolation trench, an element isolation portion is formed. To form the insulating film (or isolation film) for filling the isolation trench, for example, a mixture gas of tetraethoxysilane (TEOS) and ozone (O3) may be used. Alternatively, the insulating film may be formed by causing decomposition reaction between monosilane and oxygen through use of a high-density plasma.
The technology is described in Japanese Patent Laid-open No. 235157/1993, which discloses a technology for filling an insulating film into field regions with different widths.